The need for hydraulic or compressed air-operated actuators exists in many technical areas. Traditional actuators have a piston arranged for reciprocal motion within a cylinder, with the piston cooperating with an actuator rod connected to a component, the relative position of which is to be changed. Typically, the piston is provided with at least one piston ring to ensure no leakage of the operating fluid past the piston. In many applications, the piston rings are made of a soft, elastic material such as rubber or a plastics material. In working environments in which an actuator may be subjected to high temperatures, however, it is not possible to use materials having a low melting point. In such cases it has hitherto been necessary to employ piston rings made of, for example, spring steel.
Nevertheless, even spring steel has limitations as regards temperatures to which it may be subjected. For example, unless special precautions are taken, it is not possible to use such an actuator in the vicinity of the exhaust manifold of an internal combustion engine.